1. Field of the Invention
The present invention relates to a technique for converting an AC power supply of variable frequency and variable voltage into a DC power, and particularly to a power conversion system suitable for converting an AC voltage of the output of an inverter or an AC power generator for a non-contact current collector used with a magnetically-leviated railway, an automobile or wind power generation into a DC voltage.
2. Description of the Prior Art
A power supply system for producing a DC voltage by use of a power converter from an AC power supply of variable frequency and variable voltage finds a variety of applications including the battery charger for automobiles. An application of such a system as an onboard power supply system for a magnetically-leviated railway is also expected.
Non-contact current collectors for the magnetically-leviated railway are well known and have been disclosed in various media including "The Institute of Electrical Engineers of Japan Journal Division B", Vol. 101, No. 1 (1981), p.p. 33 to 40, "The Journal of 20th National Symposium for Cybernetic Utilization in Railways" 1983, p.p. 549 to 543, and JP-A-61-121773.
The system disclosed in IEEJ Journal, Division B, Vol. 101, No. 1 (1981), p.p. 33 to 40, is for rectifying an AC voltage generated in a current-collecting coil through a diode full-wave rectifier circuit to supply power to a load, and has no function of controlling the DC output voltage.
The system illustrated in the Journal of the 20th National Symposium for Cybernetic Utilization in Railways, 1983, p.p. 549 to 553, on the other hand, further comprises a chopper circuit (booster) in addition to all the component parts included in the aforementioned system to provide the function of controlling the DC output voltage.
Also, the system disclosed in JP-A-61-121773 comprises a power converter having a self-quenchable switching device for dampening the output drop due to the reactance on the AC power side in order to supply a greater amount of power.
In producing a DC voltage from an AC Power generator as in the automobile, a system having a diode rectifier circuit has so far been commonly used with an AC power generator having the function of controlling the field current for regulating the DC output voltage, as illustrated in "The Car Electronics Subsystem", FIG. 3, p. 146, published by Chunichi Co.
In a three-phase AC power supply having an internal impedance (inductance L and resistance R) and balanced in variable frequency and variable voltage, assume that the voltage of respective phase (hereinafter called "the source voltage") is E.sub.uo =E.sub.vo =E.sub.wo =E.sub.o, the phase current (hereinafter called "the power current") is I.sub.u =I.sub.v =I.sub.w =I, and the power factor angle is .phi.. If the high harmonics of the voltage and current and the loss of the converter are ignored, the power P available from the power supply is expressed by the equation below. EQU P=3(E.sub.o Icos.phi.-RI.sup.2) (1)
As seen from this equation, the power P becomes maximum when the equations below are satisfied. EQU Cos.phi.=1 (2) EQU I=E.sub.o/ 2R=I.sub.1 ( 3)
Even if the source current more than this is supplied, the power does not increase but rather decreases. In the process, the maximum power P.sub.max available from the power supply is given by EQU P.sub.max= 3E.sub.o.sup.2/ 4R (4)
The source current, on the other hand, has an allowable maximum value determined by the source capacity or the like. Let the allowable maximum value be I.sub.max, and the relation indicated below holds from the equation (3). EQU I=E.sub.o/ 2R.ltoreq.I.sub.max EQU E.sub.o.ltoreq. 2RI.sub.max ( 5)
In this range, the source current is required to be limited below I.sub.max. Under this condition, the maximum power P.sub.max available from the power supply is given as EQU P.sub.max= 3(E.sub.o I.sub.max -RI.sub.max.sup.2) (6)
Thus, in accordance with the source voltage, it is possible to produce the power defined by equations (4) and (6) from the power supply.
In a rectifier circuit using a switching device not quenchable by itself such as a diode or a thyristor, the reactance included in the AC power supply side causes an overlapping of commutations at the time of commutation of the device. This phenomenon reduces the power factor of the power supply equivalently. With the increase in the load current, this overlapping of commutations is further enhanced, so that the DC voltage is reduced, thereby making it impossible to supply effective power.
In the case where the commutation overlap angle is less than 60 degree in a three-phase diode full-wave rectifier circuit, for example, the DC output voltage E.sub.d is given by the equation below if the DC current is assumed to be completely smoothed. EQU E.sub.d =E.sub.do -(3/.pi.) .omega. LI.sub.d ( 7)
where E.sub.do is a no-load DC voltage, .delta. a power supply angular frequency, L an AC power supply inductance, and I.sub.d a DC current.
In equation (7), the second term on the right side represents a voltage drop due to the commutation overlapping. This voltage drop increases in proportion to the DC current and the power frequency. From the relation of equation (7), the conversion power P.sub.d is determined as EQU P.sub.d =E.sub.d I.sub.d =E.sub.do I.sub.d -(3/.pi.) .omega. LI.sub.d.sup.2 ( 8)
It will be seen that the phenomenon of the commutation overlapping reduces the conversion power P.sub.d by an amount equivalent to the second term on the right side in the above equation. Especially in the region of high source frequency where the reactance is large, power cannot be taken out effectively from the power supply.
Also, in the region of low source voltage where the DC output voltage is also low, power cannot be supplied until the DC output voltage reaches the battery voltage, and therefore the battery fails to be charged. In the meantime, the load is supplied with power only from the battery, thus increasing the burden on the battery. If a predetermined level of the DC output voltage is to be maintained, a separate boosting device or the like is necessary.
In contrast, JP-A-61-121773 discloses a system intended to dampen the drop of the output by reducing the commutation overlapping as far as possible. Even this system is unable to remove the commutation overlapping completely, and a predetermined value of the DC output voltage of this system is capable of being maintained only in a limited source voltage range, as in the case of the aforementioned diode rectifier circuit.
If a voltage-type PWM converter with a self-quenching device including a power transistor is used as a power converter, the Positive utilization of the reactance on the AC Power supply side prevents the commutation overlapping. The conventional methods of control, however, fail to take into consideration the operation under a low source voltage with the result that an attempt to collect from a power supply a current far exceeding the current value indicated by equation (3) has caused an undesirable power reduction within an allowable maximum value of the source current. In the region of high source voltage, on the other hand, a maximum value of the AC input voltage of a PWM converter primarily determined by the DC voltage, limits the range of the source voltage where the PWM converter is operable. The system disclosed in JP-A-62-210866 is known as one for solving these problems. In this known system, when the source voltage increases to a level where the input voltage approaches a maximum value, an internal control variable associated with the input voltage is corrected in accordance with the difference between the maximum value and the input voltage to prevent the input voltage from exceeding the maximum value, thereby making possible a continued operation. This system, in which the input voltage is limited below the maximum value by a complicated control loop, poses the problem of how a stable control characteristic is to be secured.